Composition comprising trifluorochloroethylene copolymer and silica and process for molding same



properties of the elastomer.

2 2,954,359 COMPOSITION COMPRISING TRIFLUOROCHLO- ROETHYLENE COPOLYMER AND SILICA AND PROCESS FOR MOLDING SAME I Lester E. Robb, Westfield, and David R. Wolf, Bloom'- field, N.J., assignors, by mesne assignments, to Minnesota Mining and Manufacturing Company, St. Paul,

Minn., a corporation of Delaware No Drawing. Filed Mar. 14, 1955, Ser. No; 494,249

Claims. (Cl. 260-41) This invention relates to perfluorochloroolefin polymers and in one of its particular aspects to the modification of the physical properties of these polymers by the use of reinforcing fillers.

Because of their unusual chemical and physical char acteristics, the fluorine containing thermoplastic polymers are widely used in numerous industrial applications where their properties are best'exploited. Among the most outstanding of the properties of the fluorine containing polymers, such as polymers of trifluorochloro- United States Patent ethylene, is chemical inertness. Thus, these polymers may be exposed to a wide variety of oxidizing,rreducing and solvent-type reagents with little or no apparent effect on the polymer. In addition to their high degree of chemical stability, these polymers possess high thermal stability, excellent electrical properties and are readily molded into various useful end items. 1

Homopolymers of the perfluorochloroolefins,such as trifiuorochloroethylene, have been developed to a high degree of commercial success due'to their unique properties as described above. alter the properties of the homopolymer by copolymeriz ing'the homopolymer with various hydrogen-containing fluorinated olefins. These copolymers of the perfluorochloroolefins have many of the desirable properties of the homopolymer, and additionally, possess characteristics which make them generally useful over and above the homopolymer. In this connection, it has been found that copolymers of the perfluorochloroolefin and a hydrogen-containing fiuorinated olefin, for instance trifluorochloroethylene-vinylidene fluoride copolymers which contain above 95 mol percent of the perfluorochloroolefin, are more thermoplastic than the homopolymer. Copolymers containing between above about 69 and below 80 mol percent of the perfluorochloroolefin are soluble in a wide variety of oxygenated organic solvents, such as esters, ketones, and cyclic ethers, and may thus be applied in. the form of coatings. Copolymers containing between about and about 69 mol percent of perfluorochloroolefin are elastomeric and therefore can be fabricated into various rubbery end items.

With reference to the above copolymers and with particular reference to the elastomeric copolymers, it is frequently desirable to employ fillers. These fillers dilute the quantity of raw materials which are needed to fabricate end items, and to a limited extent, modify the physical properties of the elastomer. In the case of elastomeric perfluorochloroolefin copolymers, a number of materials are suitable for use as fillers, such as clay, asbestos, etc. These fillers simply dilute the quantity of elastomeric which is needed in a particular end item without modifying, to a significant extent, the physical Fillers of the type usually referred to as reinforcing fillers, would greatly extend the range of utility of the perfluorochloroolefin polymers by providing a means for altering the physical characteristics of the polymer involved. Reinforcing fillers not only act as diluents, but additionally, exhibit a pronounced eifect on the characteristics of the filled material.

High tensile strengths, greater elongation, etc., canbe obtained by their use. Precisely how reinforcing fillers alter the characteristics of the material in which they It has been found possible to elastomer' systems.

7 Patented Sept. 27, 1960 the reinforcement is due to a surface absorption phenomena. Whatever the reason for reinforcement, it" is interesting to note that there is no basis for predicting or extrapolating observed behavior of a given material in'one polymer to another. For example, clays, asbestos, etc., which do not'reinforce perfluorochloroolefin polymers are widely used as:reinforcing fillers in other The 'term polymer includes both homopolymers and copolymers. It is an object of this invention toprovide reinforcing fillers for the perfluorochloroolefin polymers.

It is another object of this invention to provide an elastomeric perfluorochloroolefin polymer composition having improved physical characteristics.

It is another object of this invention to provide a process for modifying the physical properties of the perfluorochloroolefin copolymers.

It is still. another object of this invention to provide materials which alter the physical characteristics of elastomeric perfluorochloroolefin polymer systems.

. It is one of the more particular objects of this invention to improve the physical properties of elastomeric copolymers of trifluorochloroethylene and vinylidene fluoride. i

Various other objects and advantages of the present invention will become apparent to those skilled in the art on reading the accompanying description and disclosure. In generaljeach of the above objects is accomplished by employing a silica reinforcing filler in conjunction with the perfluorochloroolefin polymer. Since the copolymers to which this invention is applicable can be cured or cross-linked, it will usually be desirable to in corporate curing agents, accelerators, etc. in addition to the silica reinforcing fillers.

I As indicated previously, the copolymers to which this invention is applicable are prepared by copolymerizing a perfluorochloroolefin with a hydrogen-containing fluorinated olefin. Representative of suitable perfluoro chloroolefins, are trifiuorochloroethylene, symmetrical and unsymmetrical dichlorodifluoroethylene, 2-chloroperfluoropropene, etc.; representative of the hydrogen-containing 'fluori'nated olefins which can be used as comon omers in the copolymerization with the above described perfluorochloroolefins, are vinylidene fluoride, vinyl fluo ride, 1,2-difluoroethylene, 1,1,1-trifiuoropropene, etc. The preferred copolymer is prepared by copolymerizing trifluorochloroethylene and vinylidene fluoride and where maximum elastomeric properties are desired, by effecting the copolymerization so as to produce a copolymer con.- taining between about 20 and about 69 mol percent of the perfluorochloroolefin, e.g., trifiuorochloroethylene. I

Since the process of this invention relates to the reinforcement of polymers and not to the preparation of the polymer that is reinforced, unnecessarily detailed description of the method of preparation of the copolymer is not'warranted. 'However, for a fuller understanding of the invention, an example showing the preparation of the elastomeric copolymer is given below in which a water-suspension type recipe was used.

The preferred elastomeric copolymer may be prepared using the following water-suspension type recipe. v 2

Parts by weight Catalyst and' activator solution was prepared by dissolw ing 1 part of K S O in 20 parts of Water. In still another 20 parts of water, 0.1 part of FeSO .7H O was dissolved. 140 parts of water were next charged to a silver-lined steel bomb. The aforementioned K S O Na S O and the FeSO .7I-I O solutions were then added in succession. .The .contentsof ,the:bomb werefrozen after eachaddition. Thebomb wasthen closed and evacuated. There- :after, '645 parts of CF =CECl and 35.5 parts .of .CF ;CH were flash-distilled into it. The bomb was then rocked at room-temperature (between about.25 C. and about 35 C.) fora period of 24 hours. The residual monomer was then vented from the'bomb and a mixture ofwater and chunks of rubbery polymer were discharged. These chunks were washed with hot water to remove residual salts and were then dried in vacuo at roomtemvperature. The mol percent of CF =CFCl combined in the resulting copolymeric product was 49 percent. By varying the monomer feed ratios and the monomers. employed, any copolymer system, as described above, can be prepared.

As indicated previously, reinforcement of the perfluorochloroolefin copolymers is achievedby the use of silica. The silica can be naturally occurring or can be prepared by standard precipitation processes or by oxidation of silicon tetrachloride. Anhydrous silica and silica gel can be employed although no advantage results from the use ofthe more expensive anhydrous silica. The silica can be subjected to .various refining processes, e.g. acid extraction, which increase its purity. In this connection, it should be noted that relatively impure grades of silica can be employed provided that the impurities do not. exceed about 25 percent by weight based on the silica. Impurities which are normally present in silica, include metal oxides, such as calcium oxide, aluminum oxide,.iron oxide and salts, such as sodium sulfate and sodium chloride. However, these impurities, since they exert no reinforcing effect, tend to diminish the reinforcing effect of the silica. Other impurities can also be present provided that they do not cause degradation of the polymer. Thefree silica, described above, can be combined with a variety of organic materials which enhance the organophilic properties,rand in some instances the hydrophobic properties, of the silica. Included among the combined silicas are the silicone coated silicas. These silicone coated silicas are ,prepared by treating silica gel with a solution-of silicone polymer, preferably normally liquid, in a suitable solvent, such aslower alcohols, ketones, etc. .When silica is treated in this manner, a physical bond is believed to be developed between the silica and. the silicone polymer. Thesilicone polymers which can be employed, are those homopolymers in which the siloxane units are (CH SiO or (CH )C I-I SiO. The copolymersiloxanes may contain any combination of the above units and in addition may contain small amounts of (C H SiO, etc. Other silicone polymers can be employedalthough the above enumerated are preferred. In addition to coating the silica fillers as described above, the silica can be chemically bonded to organophilic materials. To provide a chemical bonded silicone coated silica surface, a halo silane or an amino silane is polymerized in the presence of the finely divided silica gel. The silicone polymers described above are again preferred. In addition to the silicone coated silicas, a class of compounds known as the Estersils can be employed. The Estersils are prepared by reacting an alcohol with silica. The precise mechanism is not known but it is believed that the alcohol reacts with siliceous acid (H SiO so as to form the corresponding esterified silica. The alcohols with which the silica is ester-ified, are the aliphatic alcohols havingfrom 2:18 carbon atoms and preferably from 3:12 carbon atoms. While polyhydric alcohols can be employed, the primary and secondary monohydric alcohols are preferred. Typical of the alcohols which can be employed are primary and secondary monohydric alcohols, such as ethyl, npropyl, n-butyl, n-amyl, n-hexyl, n-heptyl, n-octyl, nnonyl, ,n-decyl, n-undecyl, n-dodecyl, etc., alcohols;

branched chain primary alcohols, such as isobutyl and isoamyl alcohols; secondary alcohols such as isopropyl, .sec butyl alcohols and the alicyclic alcohols such as cyclo pentanol and cyclohexanol. The quantity of the organophilic compound employedshould be sufficient to provide .alayer, usually-monomolecular,on at least 25% .ofithe surface .of the silica and preferably on at least 50% of the surface. From the foregoing, it willbe apparent that silica jin-a variety of forms can be employedas reinforcing fillers for the polymers herein described. Detailed description of the methods of preparing the reinforcing silicas can be found in the literature. Irrespective ofthe type of silica employed, the silica should-be in finely divided form. The particle size ofthe silica should not exceed 20 microns and should preferably be below about 10 microns in order to secure maximum reinforcement. For a given type of silica, maximum reinforcement is attained with the finest particle size used.

In blending the silica reinforcing fillers of this invention with the perfluorochloroolefin polymers, between about 1 and about 50 parts by weight of silica per 100 parts of polymer, preferably between about 10 and 30 parts of silica is admixed with the polymer and thoroughly dispersed therein. For example, 100 grams of a copolymer of trifiuorochloroethylene andvinylidene fluoride (SO/50 mol ratio) are handed on a rubber mill. 20 grams of refined silica in finely divided form is then addedto the rolling bank of elastomer in the nip of the rolls. After blending, the filled elastomer is then pressed into a sheet at a temperature of about 230 F. and a pressure of about 1000 psi. The sheet has the characteristics of ;a reinforced gum rubber with the added advantage of high chemical resistance. As indicated previously, copolymers of the perfluorochloroolefins and particularly the elastomeric copolymers that is, those having a mol concentration of perfluorochloroolefin between about 20 and about 69 mol percent, are susceptible to cross-linking or vulcanization. While the use of the silica reinforcing fillers is advantageous in those instanceswhere uncuredorgllm rubber is required, in most instances it will be found preferable to incorporate curing or cross-linking agents and accelerators into the polymer.

Curing or cross-linking of the copolymer is eifected by incorporating within the copolymer a cross-linkingagent which can be a peroxy type compound, a polyfunctional amino compound ora precursor of a polyfunctional .amino compound. The peroxy type compounds include both organic and inorganic compounds which contain oxygen atoms directly linked to oxygen atoms and shouldbe stable below about 50 C. in order to avoid cross-linking during the blending operation. Among the organic peroxy compounds, are the acyl and acoyl peroxides, and hydroperoxides, such as ditertiary butyl peroxide, .dilauryl peroxide, dibenzoyl peroxide and ditertiary butyl hydroperoxide. The organic peroxy type compounds also include peresters having either organic or inorganic peroxy oxygen. The former would include such compounds as alkyl and aryl perbenzoates, while the latter would include alkyl and aryl persulfates. Among the inorganic peroxy compounds, are hydrogen and metal peroxides, such as lead, barium and zinc peroxide. Among the polyfunctional amine compounds which may be used as crosslinking agents, are ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine,.hexamethylene diamine, piperazine, 1,5-naphthalene diamine, diaminoanisole, diaminobenzoic acid (all isomers), diamino stilbene (all isomers), diarninotriphenylmethane, triaminotriphenylmethane, diaminophenol (all isomers), tetramino 3,3-dimethyl-diphenylmethane,- diarninobenzene (all isomers), triaminobenzene (all isomers), triaminobenzoic acid (all isomers), triaminophenol, 1,2-propylenediamine, 1,2,3-triaminopropane, etc. Among the precursors of amines, are the .polyisocyanates, polyisothioamine, polyamine salts, polyureas, polycarbamyl halides and polyurethanes. Precursors of amines are compounds which by their own decomposition or by reactionwith other materials under curing conditions, produce polyamines. Preferred amines are the primary aliphatic di amines. The curing or cross-linking agent is used in a 6 tween about 250 F.350 F. for between about 1 and about 72 hours at atmospheric pressure. In the case of peroxide cured recipes lower initial cures-are preferred, e.g., between about 190 F. and about 250 F. while in concentration between about 0.5 and aboutZO parts by 5 the case of amine cured stock, higher cures are preferred, weight preferably between about 3 and about 15 parts 1.e., between about 250 F. and about 300 F. Molding by weight based on 100 parts by weight of polymer. 7 r ,7 can ,be accomplished using compression, extrusion and In cross-linking theperfluorochloroolefin polymers, it injection techniques. is preferred toincorpor'ate in' addition to the above de- In order to i llustrate the process of this invention, the scribed cross-linking agents basic metal compounds which 10 following tables of data are presented. The data in the react as accelerators. Among the basic metal compounds tables illustrates the reinforcing effect of different types which can be employed as accelerators, are the basic of silica. The precipitated silica referred to in the tables metal oxides, such ,as magnesium .oxide, zinc oxide and .is commercially-available as Hi-Sil 202. This silica is lead oxide and additionally, in the case ofperoxide cured prepared by a wet precipitation process and shows the recipes, the basic lead salts, such as dibasic lead phosfollowing composition on analysis; Si0 84 percent, phite, tribasic lead sulfate and tribasic lead maleate. A1 0 4 percent, CaO 1 percent, NaCl 1 percent, Fe O Where basic lead salts are .used they are preferably used .3 percent and the balance of water. The refined silica is in combination with basic metal oxides since a synergistic commercially available .as Hi-Sil 303 and has the followeifect appears to result from the combination. Thepreing composition, SiO 88 percent, Na SO 0.1 percent, cise mechanism of the acceleration is not known. The g Q CaO 0.04 percent. The silicone coated silica is available concentration of the basic a metal compound will vary as LM-3 and is. prepared by coating silica with a linear from about 0.5 to about 30' parts by weight and preferdimethyl siloxane polymer. The esterified silica is comably from about 1 to about parts by weight for each .mercially: available as Valron Estersil. This particular 100 parts of polymer. p I V esteritied silica has a completely hydrophobic surface of V In compounding the periluorochloroolefin copolymer .25 3 butoxy groups per square millimicron on a particle of so as to effect cross-linking,'the copolymer is mixed with' mean ultimate diameter of 8-10 millimicrons. In presuitable accelerators and'curing agents. To provide reparing the recipes given in the tables below, the followinforcement, silica fillers as described above, are added to ing procedure was employed. 100 grams of elastomer the polymer. Blending of the components is carried out was placed on a two roll mill and worked until a conin suitable mechanical mixing equipment, such as two tinuous band was formed. The rolls .were. heated to a roll mills, Banbury mills and screw type plasticators. temperature of about 60 C. to hasten the formation of Since, the mechanical blending involves shearing forces the band. After the copolymerwas banded, the accelerwhich necessarily generate heat, the cross-linking or ator, silica and curing agents were slowly added, in the curing agent is usually added last. In a preferred method order given, to the rolling bank of copolymer in the nip of operation, the copolymer is introduced into the mixing of the rolls, with cutting and turning of-the copolymer equipment after which the accelerator and the silica reinas it banded on the rolls. During the blendingoperation, forcing filler, are added. When these have been thorough-j the temperature was maintained at about 100-150 F. 1,; dispersed in the copolymer, the curing agent is'added. After the addition of all the ingredients, the batch was Curing 'of the polymerfis eflected using an initial (e.g.,, I thoroughly mixed on the rolls and then sheeted out for press) cure between about-150 F. and about 350 F. 40 the molding operation which in this case involved the for a period of time between about 10 minutes and about preparation of standard A.S.T.M.' test sheets. These test 3 hours at a pressure between about 500 and about 1500 sheets were prepared by taking a sheet of stock approxp.s.i. followed by an after cure (e.g., oven cure) at beimately 10 percent thicker than that desired, and placing Table I Silicone Refined Esteri- Filler Type Control Silica Coated Silica fled Silica Silica Stock Number '.--1L .1 89 119 122 183 195 Compound:

Trifluorochloroethylene -Vinylidene Fluoride Qopolymer 50m r 10) .1. 100 100 100 100 100 line 0 e 10 10 10 10 1e lDibaslc Lead Phosphite 10 10 10 10 10 Benzoyl Peroxi 3 3 3 3 3 11-ao2. 2o 14M-3. 20 1 150-511-3031; 2o

l I Valron Ester I 2o Press Cure: Temperature, F 230 230 230 230 230 Time, hrs-. 16 3d a Pressure, psi. 1,000 1,000 1,000 1,000 1,000 Oven Cure: Temperature, F- 300 300 .300 300 300 5 J Time, rs 16 16 16 16 16 1 Physical Properties:

Alter Cure Tear Strength, p.p.i 104 370 254 402 426 Tensile Strength, p.s.i 2,170 1,410 2,790 2,250 1,600 Stress at 300%11, p.s i 820 1,215 1, 670 1,630 1,280 Percent Elongation- 450 600 450 520 5 Hardness .Shore A. 58 76 71 81 89 Aged Three bays at 400 F. Tensile Strength, p.s.i 1, 235 725 1, 295 1, 610 1, 440 1 Y Stress'at 300% E,p.s.i- 371 930 906 ,130 PercentElongation.-. 800 600 675- 5 225 I Hardness, Shore 11.... 78 84 93 Aged Nine Days at 400 F Tensile Strength, p.s.i 1,315 1,080 1,185 1,625 1,585 Stressat 300% E, p.s.i 43 870 1,130 1,440 Percent Elongation- 825 325 600 5 200 Hardness, Shore A 59 79 74 83 93 1 .j v j-' PMmdmum tensile at yield point, tensile at break-=755.

7 itV-in ,a mold. The mold was placed in a suitable press havingplatens heated to 230-260 F. The mold was then closedandthesheet formed, partially curedin thepress collecting-the data of Table ,I. In eaeh case 100 grams of copol-ymer was ,used. Theweightof theadditives :is given in grams.

Table II Isocyanate Isocyanate Amine Amine Type Cure- Filler Type Control With Control 'With I Control With -Silica l "Silica -Silica Stock Number 1 218 98 219 37 221 Compound:

Trifluoroehloroethyleneavinylidene Fluoride Copolymer (50/50 11101 ratio) 100 100 100 Zinc Oxide 5 5 5 Hi-Sil-303 Methylene bis-( i-phenyl isocyanate) 5 5 =6 Tetraethylene pentamine ,1 Press Cure:

Temp., F 260 :260 260 Time, hrs 1 1 ,1 1 -1 1 Pressure, p.s.i 1, 000 1, 000 1, 000 1,000 1,000 1,000 Oven Cure: J 7

Temp., F 212; 250 212 ,250 300 .260 Time, hrs 72 '24 16' 16' 1 16 Physical Properties:

After Curev Tensile Strength, p.s.i 800 1, 930 1,250 1, 850 1,620 2,150 Stress at 300% E, p.s.i 590 1, 670 630 1, 830 1,020 2,150 Percent Elongation 320 400 580. '300 450 300 Hardness, Shore A 61 79 60 82 80 followed by heating in an air-oven. The actual temperatures, pressures and curing times are specified in thetables.

In Table I are presented, data illustrating the use of. the sicila reinforcing fillers of this invention in a peroxide cured stock.

In order to illustrate the use of the silica reinforcing fillers of this invention in an amine, isocyanate and amineisocyanate vulcanized stock, the data of Table II is presented below. In this table refined silica (Hi4il-303) was usedin each instance, The copolymers and the compounding techniques employed were the sarne as used in As indicated previously, reinforcement of the .elasto-, meric perfiuorochloroolefin;polymers. is. achieved only. by the use of silica fillers. Thus, asbestos, vclay, .barytes, and alumina when compounded into the elastomer do not exhibit any reinforcing properties. -In-order to illustrate this phenomenon the following table is presented. .Here again, theeompounding technique was jthe vsame as described with reference to Table I. jlOO gnamsof oopolymer was empolyed. The ,weight of the...additives is given in grams.

Table III Filler Type Asbestos Clay Barytes Alumina Refined Clay Stock Number 190 191 192 193 201 Trilluorochloroethylene-Vinylidene Fluoride Oopolymer (-50 mol 7 ti 100 100 100 100 100 10 10 10 10 10 ,Dibaslc Lead. Phosphite. 10 10 10 10 10 .Benzoyl Peroxide, 3 3 3 3 3 Asbestine-3X.-. 20

20 Hydrous Alumina 2O .ASP 600 t 20 Press Cure:

Temp., F 230 230 230 230 230 Time, -hrs 36 34 Pressure p.s.i.. .t 1, 000 1,000 1,000 1, 000 1,000 Oven Cure:

fter Cure 'Iear Strength,v p.p.l. 157 161 151 197 223 Tensile Strength, p.s.i. l, 920 1,670 1, 580 2,010 2,060 Stressat 300% E, psi. 1, 270 1, 360 905 1,210 l, 560 Percent Elongation" 500 400 425 460 620 Hardness Shore A 67 67 62 66 66 Aged Three fiays at400 F.-

Tensile Strength, p.s.i 720 740 930 727 810 Stress at'300% E, p.s 740 716 470 590 871 Percent El0ngation 870 800 800 780 650 Hardness, Shore A 67 67 68 69 Aged NinejDays at400 F.-

' Tensile Strength, p.s.i 660 710 1, 000 940 1 938 Stressah 300% E, 13.5 765 855 570 850 870 Percont Elongationu 750 775 775 800 600 Hardness, Shore A 69 68 64 67 68 In addition to the above described additives, various other additives can be incorporated within the normally solid copolymer described herein. Thus, plasticizers, softeners, etc. can be added to further modify physical properties, while colored organic and inorganic pigments can be added to modify esthetic properties.

Various alterations and modifications of the invention and its aspects may become apparent to those skilled in the art without departing from the scope of this invention.

Having thus described our invention, we claim:

1. A novel composition comprising an elastomeric copolymer containing between about 20 and about '69 mol percent of trifluorochloroethylene copolymerized with vinylidene fluoride admixed with a cross-linking agent selected from the group consisting of a peroxy compound and a polyfunctional amino compound and from 1 to 50 parts by weight of silica per 100 parts by weight of copolymer, as a reinforcing filler.

2. The composition of claim 1 in which the cross-linking agent is a polyfunctional amine.

3. The composition of claim 2 in which the polyfunctional amine is tetraethylene pentamine.

4. The composition of claim 1 in which the cross-linking agent is a polyisocyanate.

5. The composition of claim 4 in which the polyisocyanate is methylene bis-(4-phenyl isocyanate).

6. The composition of claim 1 in which the cross-linking agent is an organic peroxide.

7. The composition of claim 6 in which the organic peroxide is benzoyl peroxide.

8. A novel composition comprising a copolymer containing between about 20.and about 69 mol percent of trifluorochloroethylene copolymerized with vinylidene fluoride admixed with a cross-linking agent selected from the group consisting of a peroxy compound and a polyfunctional amino compound in an amount between about 1 and about 20 parts by weight per 100 parts by weight of said copolymer and a reinforcing filler comprising silica in an amount between about 10 and about '30 parts by weight per 100 parts of said copolymer.

9. A novel composition comprising a crosslinked copolymer containing between about 20 and about 69 mol percent of trifluorochloroethylene copolymerized with vinylidene fluoride, said copolymer crosslinked with a crosslinking agent selected from the group consisting of a peroxy compound and a polyfunctional amino compound and from 1 to parts by weight of silica per parts by weight of copolymer, as a reinforcing filler.

10. A process for reinforcing copolymers containing between about 20 and about 69 mol percent of trifluorochloroethylene copolymerized with vinylidene fluroide which comprises admixing from 1 to 50 parts by weight of silica per 100 parts by Weight of copolymer and a crosslinking agent selected from the group consisting of a peroxy compound and a polyfunctional amino compound and molding under pressure the reacting mixture at a temperature between about and about 350 F. and at a pressure between 500 and 1500 p.s.i.

References Cited in the file of this patent UNITED STATES PATENTS 2,367,880 Lindh Jan 23, 1945 2,456,621 Cheney Dec. 21, 1948 2,468,054 Ford Apr. 26, 1949 2,631,998 Pearson Mar. 17, 1953 2,732,357 Sprung Ian. 24, 1956 OTHER REFERENCES Rubber Age, September 1954, page 884. India Rubber World, August 1949, pages 577-581 and 586. 

